Audacity and enthusiasm count for a lot in filmmaking, and this flick has plenty of both.

Writer/director Joe Cornish, crafting an impressive big-screen debut after some work in British TV, makes the most of a modest budget and delivers a rip-snortin’ action comedy that hits the ground running and never lets up.

“Attack the Block” will be embraced by fun-loving genre fans who enjoyed the blend of giggles and grue found in “Shaun of the Dead.” And while Cornish’s film isn’t quite as gory, this cheeky saga has its moments; the faint of heart should proceed with caution.

Cornish works a lot into his alternately whimsical and savage script: intriguing character dynamics, a clever understanding of reproductive biology — don’t worry, that’ll make sense in context — and even some perceptive social commentary. This is a tale of stepping up to the plate: of heroes so unlikely that they’re basically … well … thugs.

The film opens as trainee nurse Sam (Jodie Whittaker) walks home late one night: not the smartest move, since she lives in an inner-city South London tower block. No surprise, then, when she’s mugged by a quintet of masked, hooded teenage thugs, led by the knife-wielding Moses (John Boyega).

Sam is saved by an unlikely interruption: a bright meteorite that smashes into a nearby parked car. She flees; Moses and his crew investigate, only to be attacked by a small but vicious something. Moses, sensing a possible loss of face, tracks the creature to a small shed and kills it.

So far, Cornish’s approach has been gritty, scary and mean. But now the tone softens, as the boys shed their hoods — revealing most of them to be much younger and “smaller” than expected — and drag the alien carcass to the top of the block, making sure that everybody sees how they’ve defended their “territory.”

But nobody believes that the dead thing is real. The local drug lord — a truly dangerous, gun-toting psychopath dubbed Hi-Hatz (Jumayn Hunter) — thinks it’s just a movie-prop puppet. Veteran stoner Ron (Nick Frost), who runs a cannabis-growing farm on the council house’s top floor, can’t really be bothered to venture an opinion.

The only one who does regard the creature as something real is posh pretender Brewis (Luke Treadaway), a well-educated geek from some nearby university, who’s visiting only to score some weed … and nobody cares about his opinions.

At that moment, a second, much larger wave of meteorites strikes all over the neighborhood. Expecting more small creatures like the first, Moses leads his crew outside for battle. But these creatures are nothing like the first one; these are bigger, faster and much, much nastier.

Elsewhere, Sam is being driven around the neighborhood by police officers, in the hopes that she’ll spot the kids who attacked her. She does, just as Moses and his friends are fleeing these new alien attackers. Sam quickly discovers that her previous encounter pales in the face of this much greater, shared threat … and soon our half-dozen or so protagonists are trapped within their own tower block, doing their best to stave off a veritable army of dark, half-seen, razor-toothed nightmares.

Cornish gets considerable mileage out of his modest $13 million budget, starting with the marvelously menacing creatures, which remain scarier because they’re never quite fully revealed. (At long last: a director who understands the power of subtlety!) But none of this would matter a jot if we didn’t bond with the characters, and that’s the best part of Cornish’s approach: He gets us to like people who are — at least initially — far from likeable.

They’re also far from intelligible, thanks to an impenetrable mix of thick British accents and inner-city argot … and this is deliberate. Cornish, intrigued by the similarly opaque “droogs” in “A Clockwork Orange,” and the verisimilitude of dialect used in books such as “The Color Purple” or “Butcher Boy,” uses this technique to make Moses and his crew more frightening when Sam first encounters them. We simply don’t understand what these kids are saying, which adds to the menace of the scene.

But as the film continues, Cornish has his actors speak more slowly and distinctly, and we also begin to absorb their jargon; we grow more comfortable in their presence, just as Sam — eventually — is forced to.

Once the masks and hoods are down, as well, they become distinct individuals. Boyega gets the most screen time, which allows him to establish Moses’ contradictions: The boy is stoic, strong and silent … but he’s also willing to stand back and reconsider a situation. Most crucially, the surface swagger is a pose, as Sam discovers when she investigates his flat.

Alex Esmail is a hoot as the aptly named Pest: a sharp-tongued motormouth who serves as the crew’s demolitions “expert” … albeit only with fireworks. Leeon Jones plays Jerome: more studious than the others, and likely a dedicated student if he hadn’t fallen amongst these companions for self-preservation.

Sammy Williams and Michael Ajao make quite an impression as Probs and Mayhem, a pair of 9-year-old gang wannabes who resent the condescending attitude of the older kids, and decide to do something about it.

Aside from Boyega’s Moses, most of the dramatic heft comes from Whittaker — well remembered as the teen who tantalized Peter O’Toole in 2006’s “Venus” — who has the tough job of selling Sam’s decision to fall in with gangbangers who (she believes) easily might have killed her a few hours earlier. As a result, she also becomes the token “adult” and voice of reason … assuming the others will listen to her. Whittaker deftly handles Sam’s crucial switch of allegiance.

Cornish makes excellent use of the disconcerting, dilapidated setting: the council house with its long, dark hallways that are illuminated only when somebody triggers a sensor that activates the lights. Sometimes.

Yes, this film’s modest budget betrays it at times. The film stock is grainy, and cinematographer Thomas Townend occasionally keeps things a bit too dark; we need to see enough to appreciate the dangers, or lack thereof. But these are minor quibbles that certainly won’t interfere with the thrill ride Cornish orchestrates so well.

He’s just as talented as Carpenter was, this early in his own career. Let’s hope Cornish makes better use of the opportunities certain to come his way, in the wake of this engaging debut.

— Read more of Derrick Bang’s film criticism at http://derrickbang.blogspot.com. Comment on this review at www.davisenterprise.com